In order to transfer data at a high speed in a state where a large number of nodes are connected to one another, a propagation delay time of wiring cannot be ignored. In particular, in a DDR-SDRAM (double data rate synchronous DRAM), an operating frequency of data is twice as large as that of an address, and the effect of noises reflected from a branched wiring on a bus wiring makes high speed difficult. Examples of methods for solving the above problem include “non-contact bus” in JP 07-141079A (U.S. Pat. No. 5,638,402), “directional coupling memory module” in JP 2001-027918A (U.S. Ser. No. 09/570,349), and “directional coupling bus system” in JP 2001-027987A (U.S. Ser. No. 09/569,876).
FIG. 2 shows the structure of the directional coupling bus disclosed in JP 07-141079A.
In the above method, data transfer between two nodes is conducted by using a backward crosstalk, that is, the transformation of from an NRZ signal to an RZ signal through a directional coupler. That is, this is a technique in which transfer between a bus master 10-1 and slaves 10-2 to 10-4 is performed by using two lines, that is, the backward crosstalk between a wiring 20 and wirings 20-1 to 20-4. This technique is suitable for transfer between a bus master 1 and the slaves 10-1 to 10-4, that is, suitable for data transfer between a memory and a memory controller. In this example, directional couplers that are connected to the bus are identical in the configuration with each other, and the coupling coefficients (KB) and the coupling lengths L1 to L4 of those directional couplers are also constant.
Subsequently, in a conventional example, “directional coupling bus system” of JP 2001-027918A, a main line 20 is folded back to provide the directional coupler with a multilayer structure, thereby realizing a high density. Similarly, the coupling lengths of the directional couplers are constant in this example.
In the “directional coupling memory module” of JP 2001-027987A, a wiring (main line) from a memory controller is drawn in memory modules, and a directional coupler is structured within each of the memory modules. Similarly, in this example, the memory modules that are connected to the memory bus are identical in the configuration with each other, and the coupling coefficients (KB) and the coupling lengths of the directional couplers within the memory modules are also constant.
The above conventional examples have the feature that the lengths of the directional couplers are constant. The reasons are as below.
In general, when a rise time of a drive pulse is shorter than a propagation delay time of reciprocating of the directional coupler, the directional coupler generates an amount of backward crosstalk signal not depending on the coupling length. For that reason, a ratio of an input voltage to a backward crosstalk voltage gets constant not depending on the length. In FIG. 2, if attenuation when a drive waveform from an MC 1 propagates on the wiring 20 can be ignored, the production of the backward crosstalk signals by the wirings 20-1 to 20-4 becomes constant.
For that reason, in the conventional art, when it is assumed that the directional coupler lengths that generate the respective crosstalks are L1 to L4, L1=L2=L3 L4 is constant, and the wiring intervals of the directional couplers are also identical with each other in order to realize the same coupling coefficients of the directional couplers. The directional couplers that have the constant wiring intervals and lengths generate substantially the same signal amount with respect to any bus slaves.
As described above, in the conventional art using the directional couplers, the coupling lengths of the directional couplers within the bus to be used are constant, and an interval between two lines which determine the coupling coefficients (Kb) is also constant.